Building block



FIG. 1 is a top, front, right perspective view of a building block;

FIG. 2 is a bottom, rear left perspective view thereof;

FIG. 3 is a top plan view thereof;

FIG. 4 is a right side view thereof, the left side view being a mirror image thereof;

FIG. 5 is a bottom plan view thereof;

FIG. 6 is a front plan view thereof; and,

FIG. 7 is a rear side view thereof.

The broken lines depict portions of the building block that form no part of the claimed design. 

CLAIM The ornamental design for a building block, as shown and described. 